U.S. Pat. No. 3,639,254 and U.S. Pat. No. 5,834,053 have already disclosed thiogallates, the emission spectra of which lie in the blue or green spectral region. These phosphors follow the formula AGa2S4, where A represents at least one element from the group of the alkaline-earth metals, in particular Ca, Ba, Sr or Zn. Activators are europium, lead or cerium. However, for applications which require a high light efficiency (e.g. illumination engineering), the emission efficiencies of said phosphors are too low. This emission efficiency is expressed by what is known as the quantum efficiency QE (ratio of number of quanta emitted to the number of excitation quanta absorbed). Typical quantum efficiencies for said phosphors are between 60% and 70%.
WO 98/18721 has disclosed an electroluminescent phosphor selected from the group of the thiometallates, with Sr or another alkaline-earth metal as divalent cation, Ga, Al or In acting as the trivalent cation. In particular, this document describes a production process which retains a certain amount of residual oxygen.
It is an object of the present invention to provide a thiometallate phosphor which has a quantum efficiency which is as high as possible for a predetermined emission wavelength.
According to the invention, the composition of the phosphor is selected in such a way that the ratio of divalent ions A to trivalent ions B, working on the basis of the general empirical formula AB2S4, differs from the ratio A:B=1:2. The concept of the invention can also be expressed in a different way if the thiometallates of the original empirical formula AB2S4 are written as a product of the components AS and B2S3 in the form ASxc2x7B2S3. The ratio of the component AS to the component B2S3 is described below by the factor w=B2S3/AS. The overall result is that the thiometallate is represented as (AS)xc2x7w(B2S3) . It has been found that phosphors having the composition (AS)xc2x7w(B2S3) provide higher quantum efficiencies than phosphors with the composition w=1 both in the range 0.8 xe2x89xa6wxe2x89xa60.98 and in the range 1.02 xe2x89xa6wxe2x89xa61.2.
The combination of various cations of type A and B makes it possible to achieve different emission wavelengths and color loci and to adapt them to the particular application. For an efficient (""""brightxe2x80x3) phosphor, there must additionally be a low reflection in the excitation range and a high quantum efficiency. Mg, Ca, Sr, individually or primarily in combination, are suitable as cation A. It has proven particularly appropriate to use all three metals together. Europium or cerium are suitable activators which partially replace A. It is preferable for Ga or Al or Y to be used as cation B. The gallium may in this case in particular be partially replaced (up to 10 mol%) by aluminum. The dopant D (D=Eu and/or Ce) is counted completely as part of the sub-component AS, i.e. represented in full the formula is Al-tDtS.
Phosphors having the composition (AS)xc2x7w(B2S3), where A=MgaCabSrcEut, with a+b+c+t=1, where the following ranges apply: 0.4 xe2x89xa6axe2x89xa60.7; 0.1 xe2x89xa6bxe2x89xa60.4; 0 xe2x89xa6cxe2x89xa60.4; 0.01 xe2x89xa6txe2x89xa60.1 and B=(GaxAlyYz)2 with x+y+z=1 and 0.9xe2x89xa6xc3x97xe2x89xa61 and Oxe2x89xa6yxe2x89xa60.1 and Oxe2x89xa6zxe2x89xa60.1 and 0.8xe2x89xa6wxe2x89xa60.98 or 1.02xe2x89xa6wxe2x89xa61.25, preferably wxe2x89xa61.2, have particularly high quantum efficiencies. c is preferablyxe2x89xa60.01.
A production process employs the following steps:
a) production of a suspension of nitrates corresponding to the desired composition;
c) drying of this suspension to a residual moisture content of  less than 1% by weight at Txe2x89xa6300xc2x0 C., in order to produce a finely dispersed nitrate mixture;
c) milling of the nitrate mixture in a mortar mill at room temperature for 10 min to 60 min, preferably 15 to 25 min;
d) pyrolysis of the milled nitrate mix at 500-700xc2x0 C., preferably at 600xc2x0 C., in an Ar or N2 atmosphere in order to produce a finely dispersed metal oxide mixture of the desired composition;
e) initial reaction of the metal oxide mixture at 800-1000xc2x0 C., preferably 900-950xc2x0 C., in flowing H2S or CS2 atmosphere or combinations thereof for 1-6 hours, preferably 4 hours;
f) milling the reaction product as in step c;
g) second reaction at 800-1000xc2x0 C., preferably 900-950xc2x0 C., in a flowing H2S or CS2 atmosphere or combinations thereof for 1-6 h, preferably for 2 h.
In steps e) and g), the quantitative flow rate is preferably 50-500 ml/min, ideally 120 ml/min, and the gas atmosphere preferably comprises HsS or CS2 and Ar or N2 as carrier gas, with 10-50% of H2S or CS2 or mixtures thereof, preferably 30% of H2S or CS2 or mixtures thereof.
Gradual heating up to the reaction temperature is carried out in steps e) and g), preferably at a rate of 0.5-20 K/min, ideally 10 K/min.
Moreover, in steps e) and g) gradual cooling is carried out after the reaction, preferably at a rate of 0.5-20 K/min, ideally 10 K/min.
The phosphors according to the invention are particularly suitable for use in UV-emitting or blue-emitting LEDs for color conversion. They can be used individually or in combination with other phosphors, in particular in combination with other phosphors according to the invention. Plasma displays are another possible application. For this purpose too, the phosphors may be used individually or in combination with other phosphors, in particular in combination with other phosphors according to the invention, in order to convert the short-wave plasma discharge radiation into visible light.